gdbsupport, gdb: add read_text_file_to_string, use it in linux_common_core_of_thread

I would like to add more code to nat/linux-osdata.c that reads an entire
file from /proc or /sys and processes it as a string afterwards.  I
would like to avoid duplicating the somewhat error-prone code that reads
an entire file to a buffer.  I think we should have a utility function
that does that.

Add read_file_to_string to gdbsupport/filestuff.{c,h}, and make
linux_common_core_of_thread use it.  I want to make the new function
return an std::string, and because strtok doesn't play well with
std::string (it requires a `char *`, std::string::c_str returns a `const
char *`), change linux_common_core_of_thread to use std::string methods
instead.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I1793fda72a82969c28b944a84acb953f74c9230a

gdb/nat/linux-osdata.c

@@ -62,49 +62,39 @@ int
 linux_common_core_of_thread (ptid_t ptid)
 {
   char filename[sizeof ("/proc//task//stat") + 2 * MAX_PID_T_STRLEN];
-  char *content = NULL;
-  char *p;
-  char *ts = 0;
-  int content_read = 0;
-  int i;
   int core;
 
   sprintf (filename, "/proc/%lld/task/%lld/stat",
 	   (PID_T) ptid.pid (), (PID_T) ptid.lwp ());
-  gdb_file_up f = gdb_fopen_cloexec (filename, "r");
-  if (!f)
+
+  gdb::optional<std::string> content = read_text_file_to_string (filename);
+  if (!content.has_value ())
     return -1;
 
-  for (;;)
-    {
-      int n;
-      content = (char *) xrealloc (content, content_read + 1024);
-      n = fread (content + content_read, 1, 1024, f.get ());
-      content_read += n;
-      if (n < 1024)
-	{
-	  content[content_read] = '\0';
-	  break;
-	}
-    }
-
   /* ps command also relies on no trailing fields ever contain ')'.  */
-  p = strrchr (content, ')');
-  if (p != NULL)
-    p++;
+  std::string::size_type pos = content->find_last_of (')');
+  if (pos == std::string::npos)
+    return -1;
 
   /* If the first field after program name has index 0, then core number is
-     the field with index 36.  There's no constant for that anywhere.  */
-  if (p != NULL)
-    p = strtok_r (p, " ", &ts);
-  for (i = 0; p != NULL && i != 36; ++i)
-    p = strtok_r (NULL, " ", &ts);
+     the field with index 36 (so, the 37th).  There's no constant for that
+     anywhere.  */
+  for (int i = 0; i < 37; ++i)
+    {
+      /* Find separator.  */
+      pos = content->find_first_of (' ', pos);
+      if (pos == std::string::npos)
+	return {};
 
-  if (p == NULL || sscanf (p, "%d", &core) == 0)
+      /* Find beginning of field.  */
+      pos = content->find_first_not_of (' ', pos);
+      if (pos == std::string::npos)
+	return {};
+    }
+
+  if (sscanf (&(*content)[pos], "%d", &core) == 0)
     core = -1;
 
-  xfree (content);
-
   return core;
 }
 

gdbsupport/filestuff.cc

@@ -501,3 +501,40 @@ mkdir_recursive (const char *dir)
       component_start = component_end;
     }
 }
+
+/* See gdbsupport/filestuff.h.  */
+
+gdb::optional<std::string>
+read_text_file_to_string (const char *path)
+{
+  gdb_file_up file = gdb_fopen_cloexec (path, "r");
+  if (file == nullptr)
+    return {};
+
+  std::string res;
+  for (;;)
+    {
+      std::string::size_type start_size = res.size ();
+      constexpr int chunk_size = 1024;
+
+      /* Resize to accomodate CHUNK_SIZE bytes.  */
+      res.resize (start_size + chunk_size);
+
+      int n = fread (&res[start_size], 1, chunk_size, file.get ());
+      if (n == chunk_size)
+	continue;
+
+      gdb_assert (n < chunk_size);
+
+      /* Less than CHUNK means EOF or error.  If it's an error, return
+         no value.  */
+      if (ferror (file.get ()))
+	return {};
+
+      /* Resize the string according to the data we read.  */
+      res.resize (start_size + n);
+      break;
+    }
+
+  return res;
+}

gdbsupport/filestuff.h

@@ -129,4 +129,8 @@ extern bool is_regular_file (const char *name, int *errno_ptr);
 
 extern bool mkdir_recursive (const char *dir);
 
+/* Read the entire content of file PATH into an std::string.  */
+
+extern gdb::optional<std::string> read_text_file_to_string (const char *path);
+
 #endif /* COMMON_FILESTUFF_H */